The primary purpose of a lubricant is to separate moving surfaces to minimize friction and wear. The first known example of lubrication is the use of tallow to lubricate chariot wheels. Although Leonardo daVinci discovered the fundamental principles of friction and lubrication, widespread understanding of the science of lubrication did not develop until the latter part of the nineteenth century.
Lubrication can be effected by several different methods, ranging from complete separation of moving surfaces by a fluid lubricant, through partial separation in boundary lubrication, to dry sliding where solid material properties and surface chemistry dominate.
In fluid-film lubrication, the load is supported entirely by pressure within the separating-fluid film. This film pressure is frequently generated by the relative motion of the surfaces involved, which pumps the lubricant into a converging, wedge-shaped zone.
As the operating conditions become more severe, a point is eventually reached where the oil-film support can no longer carry the load completely. High spots, or separation, of the mating surfaces then must share in load support with the lubricant, and the lubrication shifts from full-film, with a coefficient of friction of about 0.001, to mixed-film, to complete boundary lubrication wherein the coefficient of friction increases to 0.1-0.3. This shift from full-film to boundary lubrication may result from any one or a combination of the following conditions: high load, low speed, low viscosity lubricant, misalignment, high surface roughness, or inadequate supply of lubricant. With boundary lubrication, chemical additives in the lubricating oil and chemical, metallurgical, and mechanical factors involving the two rubbing surfaces determine the extent of wear and the degree of friction.
Under boundary conditions of lubrication, metal contact through the oil film results in junctions of asperities and subsequent metal tearing on a microscopic scale. With increasing loads, more of these contacts occur, resulting in more plastic deformation, higher temperatures, and welding. Seizure eventually occurs on a gross and devastating scale. Hyped gears, such as in automobile differentials, are particularly susceptible to this type of damage, since these gears impose severe sliding conditions in combination with high contact stress. The intense heat leads to very high surface temperatures and ineffectiveness of the organic lubricant film that normally is present. Extreme pressure lubricants have been developed to deal with these conditions, which lubricants contain additives that react at the high contact temperatures to form low melting, inorganic lubricant films on the metal surfaces and thereby prevent massive welding and breakdown. These additives generally consist of sulfur, chlorine, phosphorus, and lead compounds that act either by providing layers of low shear strength to minimize metal tearing of by serving as fluxing agents that contaminate the metal surface and prevent welding. Because these extreme pressure additives are effective only by chemical action, use of these additives should generally be avoided to minimize possible corrosion difficulties in any apparatus where they are not strictly necessary.
Lubricating oils from petroleum generally consist of complex mixtures of hydrocarbon molecules. These generally range from low viscosity oils with molecular weights as low as 250 to very viscous lubricants with molecular weights as high as about 1,000. The physical properties of the lubricating oils, such as viscosity, viscosity-temperature-pressure characteristics, and performance, depend largely on the relative distribution of paraffinic, aromatic, and alicyclic (naphthenic) components.
For a given molecular size, the paraffins have relatively low viscosity and density and higher freezing points as compared to the other types of petroleum lubricants. Paraffinic oils have low oxidation resistance unless properly inhibited, in which case they have high stability with little tendency for sludging. Although the aromatic compounds are relatively stable to oxidation, they form insoluble black sludges at high temperatures. Aromatic oils also change viscosity rapidly with temperature, high density, and a darker color. Alicyclic oils have a low pour point, a low order of oxidation stability, and other physical properties that are intermediate those of the paraffins and aromatics. Almost all of the oils called paraffinic oils are composed of both paraffinic and alicyclic structures, with only a minor proportions of aromatics. When stabilized with an oxidation inhibitor, alicyclics offer non-sludging oils that are satisfactory for almost any type of lubricating purpose.
When petroleum crude oils are distilled, the lower boiling gasoline, kerosenes, and fuel oils are removed first, and the lubricating-oil fractions are divided by boiling point range into several grades of neutral distillates and a more viscous residue sometime called a cylinder stock. Subsequent refining steps remove undesirable aromatics and the minor proportion of sulfur, nitrogen, and oxygen compounds present. Hydrogen treatment at high pressure and in the presence of a catalyst has become the most popular refining method. Very mild hydrofining primarily involves removal of color and some nitrogen and sulfur compounds, whereas severe hydrofining or hydrocracking alters the chemical structures to convert aromatics to paraffins and alicyclics.
Low temperature filtration is often used to remove paraffin wax and thereby decrease the pour point of the oil. Lubricating oils are made by blending one or more refined oil stocks of the desired viscosity with the additives required for the expected service conditions.
In recent years, the evolution and technical progress in all types of internal combustion engines have led to higher and higher horsepower and greater efficiency. The lubricants used in these engines must form a stable and oily film, which at low temperatures will facilitate starting of the engine even in cold weather. Additionally, these lubricants must perform well at the higher operating temperatures of the newer engines in order to avoid piston fouling, ring groove plugging and lacquering, deposit formation, and the like, which lead to a drastic reduction of power output and often results in expensive damage to the engine. Furthermore, the exhaust gases resulting from fuel combustion together with a part of the lubricant must be clean and have a minimum of odors.
The addition of small amounts of certain materials to natural and synthetic lubricating oils to modify their properties in desirable ways is well known to those skilled in the art.
Turbine oils are the primary products used for circulating systems, and are the common choice for steam turbines, steel mills, paper ills electric motors, and hydroelectric generators. These oils commonly contain rust and foam inhibitors, and an oxidation inhibitor to extend the life of the oil. Hydraulic oils are developed for general use in hydraulic mechanisms and circulating systems characteristic of factory machine tools. They commonly contain a zinc dithiophosphate additive to minimize wear in high pressure hydraulic pumps. General purpose oils with no additives are used to control expenses in once-through lubrication, and can be applied by mist, drip feed, and the like in factory machines.
For gears, the SAE automotive lubricants classified between 5W and 50W are generally used for both automotive and industrial applications. Industrial gears generally use the American Gear Manufacturers Association grades of 2EP through 8A EP. These contain a variety of sulfur, phosphorus, chlorine, lead, and tallow-type additives to minimize scuffing and wear.
Most high quality oils contain organic compounds containing sulfur, nitrogen, phosphorus, and/or alkylphenols to retard the oxidation of the oils. Oil oxidation is a chain reaction involving oxygen from the air in hydroperoxide formation which leads to the formation of organic acids and other oxygenated products. Added inhibitors and some naturally occurring aromatic petroleum components appear to interrupt the chain reaction by combining with the hydroperoxide; this action delays formation of varnish, sludge, and acids for extended operating periods and minimizes corrosion problems with lead, zinc, cadmium, and copper-containing alloys which are corroded by organic acids in oxidized oils.
Rust inhibitors are surfactant materials that are preferentially adsorbed as a film on iron and steel surfaces to protect them from attacks by moisture. For mild conditions where a small amount of water is present in a large quantity of circulating oil, mildly polar organic acid, such as alkyl-succinic acids, and organic amines are often used. For the severe conditions encountered in shipping machinery, in extended storage, or in outdoor weather, more strongly adherent organic phosphate, polyhydric alcohols, and sodium and calcium sulfonates are used. When incorporated in vapor-space inhibited oils, cyclohexylamine and related amines with modest vapor pressure provide rust protection above the oil level during extended shutdown periods.
Antiwear agents are used to produce a surface film by either a chemical or physical adsorption mechanisms to minimize friction and wear under boundary-lubrication conditions. The compounds used for improved lubrication under boundary-film conditions are compounds containing oxygen, such as fatty acids, ester, and ketones; compounds containing sulfur or combinations of oxygen and sulfur, organic chlorine compounds such as chlorinated wax; organic sulfur compounds such as sulfurized fats and sulfurized olefins; compounds containing both chlorine and sulfur; organic phosphorus compounds such as tricresyl phosphate, thiophosphates, and phosphites; and organic lead compounds.
For extreme rubbing conditions where severe metal-to-metal contact is encountered, active sulfur, chlorine, and lead compounds have traditionally been used. In localized metallic contacts of high spots on the rubbing surfaces, these additives react to form low shear strength surface layers, such as lead sulfide, iron chloride, or iron sulfide. The surface layer prevents destructive welding, excessive metal transfer, and severe surface breakdown. Automotive hyped gears, slideways of machine tools, and various metal-cutting operations are representative of the types of application for these extreme pressure lubricants.
Oil detergents are conventionally used at concentrations of about 2-20% by weight to prevent high temperature deposits on internal combustion engine parts of oil-insoluble sludge, varnish, carbon, and lead compounds. The detergents act by adsorbing on insoluble particles, thereby maintaining them as a suspension in the oil to minimize deposits and to maintain cleanliness of rings, valves, and cylinder walls. Dispersants serve the same function in engines that are operated at relatively low engine temperatures that occur in short trips and in stop-and-go driving. Among the detergents that are in substantially commercial use are sulfonates, the calcium and barium salts of petroleum mahogany sulfonic acids and long-chain, alkyl-substituted aromatic sulfonic acid; phosphonates and thiophosphonates; polyolefins of about 500-2,000 molecular weight reacted with phosphorus pentasulfide and conversion of the resulting thiophosphonic acid to an alkaline earth metal salt; phenolates, calcium and barium salts of alkyl phenols, alkylphenol sulfides, and alkylphenol-aldehyde condensation products; and calcium and zinc alkyl-substituted salicylates.
Ashless dispersants are used to prevent formation of cold sludge in gasoline engines under stop-and-go driving conditions. Among these dispersants are reaction products of alkylsuccinic anhydrides with amines; polybutene treated with P2S5, steam, and ethylene oxide; polymers containing oxygen-or nitrogen-bearing comonomers, such as alkyl methacrylate-dimethylaminoethyl methacrylate copolymers, alkyl methacrylate-N-vinylpyrrolidone copolymers, and vinyl acetate-dialkyl fumarate-maleic anhydride copolymers.
A number of prior workers have sought to provide lubricating compositions which lubricate while protecting the metallic parts to be lubricated. All of these prior compositions have been based primarily on lubricating oils per se.
Barnum, in U.S. Pat. No. 2,369,740, discloses an anticorrosive for incorporation into a neutral vehicle such as normally liquid or normally solid hydrocarbons, alcohols, esters, and the like, comprising a dicarboxylic ether acid having at least 6 carbon atoms. Although these materials are anticorrosive, there is no indication that they are lubricating compositions in themselves. Moreover, the dicarboxylic ether acids are incorporated into the carriers in amounts ranging from about 0.001% to about 5% by weight.
Watkins, in U.S. Pat. No. 2, 292,308, discloses compounded lubricating oil compositions consisting essentially of a petroleum lubricating oil and a metal salt of an alkyl mono-ester of an alkenyl substituted succinic acid, or a mixture of a normal and a basic metal salt of an alkyl monoester of an alkenyl substituted succinic acid. These esters and salts are added to the lubricating oil in amounts ranging from about 0-0.5% to about 3% by weight of the oil, or even higher.
Bosniack et al, in U.S. Pat. No. 3,719,600, disclose a lubricant composition comprising a lubricating oil and a corrosion inhibiting amount of a polycarboxylic acid containing at least four non-carboxylic carbon atoms and more than two carboxyl groups. The polycarboxylic acids are present in amounts generally ranging from about 0.001 to about 1.0 parts acid per 100 parts by weight of oil.
Souillard et al, in U.S. Pat. No. 3,953,179, disclose lubricating compositions for two-stroke internal combustion engines comprising 90 to 97% by weight of a lubricant mixture comprising 15 to 80% by weight of a polymer selected from the group consisting of hydrogenated and non-hydrogenated polybutene, polyisobutylene, and mixtures thereof, with a mean molecular weight ranging from 250 to 2,000, and 0.5 to 10% by weight of a triglyceride of an unsaturated aliphatic carboxylic acid containing 18 carbon atoms, and the remainder being a lubricating oil, and 3 to 10% by weight of conventional lubricating oil additives for two-stroke engines.